RATES AND GLOBAL BIOGEOCHEMICAL CONSEQUENCES OF MICROBIAL ACTIVITY IN SUBSEAFLOOR SEDIMENTS

Our studies of chemical fluxes in subseafloor sediments of several Ocean Drilling Program (ODP) sites indicate that predominant electron acceptors, rates of net respiration and rates of net methane production vary predictably from site to site with organic flux from the surface ocean. Detailed study of open-ocean ODP Site 1226 indicates that gross respiration of dissolved inorganic carbon is in approximate balance with the net rate of electron acceptor reduction; this result indicates that the net rate of electron acceptor reduction closely matches the gross rate of electron acceptor reduction. In combination with cell counts, our results indicate that subseafloor communities at open-ocean sites respire at per-cell rates that are orders of magnitude slower than communities in published culturing experiments or shallow coastal sediments. Although subseafloor microbial activities proceed at very slow rates, they can significantly affect ocean chemistry and climate. The potential effect of subseafloor methane on climate, when catastrophically released, is well known. The effect of subseafloor sulfate reduction on ocean chemistry and atmospheric pCO₂ may be even more significant on geologic timescales but, to our knowledge, has not been previously described. In short, most sulfate reduced in open-ocean sediments is ultimately deposited in the sediment as pyrite. This process of pyrite burial affects ocean alkalinity, pH and atmospheric pCO₂ by removing strong acid from the ocean. For example, a one-millimolar increase in oceanic sulfate concentration due to decreased pyrite burial will increase atmospheric pCO₂ by a factor of four. In contrast, a one-millimolar decrease in oceanic sulfate concentration due to increased pyrite burial will drive atmospheric pCO₂ to nearly zero. We will describe possible feedback mechanisms and Earth historical implications of this process.